Cisco 3825 - Manual (Page 14)

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Cisco 3825 Mobile Wireless Edge Router Software Configuration Guide
OL-15667-03
Chapter 1 Overview of the Cisco 3825 Mobile Wireless Edge Router
Introduction
users. Each DS0 bearer channel carries up to four sub-multiplexed 16 kbps channels, termed sub-rate
DS0s. The voice and data bearer traffic is carried over the sub-rate DS0s in Transcoder and Rate Adaptor
Unit (TRAU) frames in accordance with 3GPP TS 08.60 v8.2.1, “In-band control of transcoders and rate
adaptors for Enhanced Full Rate (EFR) and full rate traffic channels.” There are several types of TRAU
frames: full-rate (FR) or enhanced full-rate (EFR) GSM vocoder frames; Adaptive Multi-Rate (AMR)
vocoder frames; silent speech frames; and OAM frames. When a sub-rate DS0 is assigned to a call,
TRAU frames are generated in accordance with 3GPP TS 08.60 v8.2.1, “In-band control of transcoders and
rate adaptors for EFR and full rate traffic channels.” When a sub-rate DS0 is idle, that is, not assigned to a
call, a repeating idle pattern is transmitted in accordance with 3GPP TS 08.54 v8.0.1, “Base Station
Controller-Base Transceiver Station (BSC-BTS) interface; Layer 1 structure of physical circuits.”
The transcoder and rate adaptation control function that specifies the TRAU frames provides several
opportunities to optimize the Abis interface, and thus optimize the backhaul bandwidth efficiency. For
example, when Discontinuous Transmission (DTX) is employed over the air interface, the TRAU frames
that are transported on the Abis interface contain standardized redundant bit patterns, known as idle
(silent) speech frames (FR and EFR) or “no data” frames (AMR), whenever a voice user is silent
(typically 40-60% of the time). As another example, bearer channels that are not assigned to calls each
carry known idle bit patterns on the Abis interface as mentioned previously. Thus, even though no radio
transmissions are made during silent and idle periods, redundant information is nevertheless transported
across the backhaul network thereby unnecessarily consuming precious bandwidth.
Cisco Pseudowire Emulation Edge-to-Edge (PWE3)
PWE3 is a mechanism that emulates the essential attributes of a service, such as ATM or EI/T1 (see
Figure 1-3 ). The required functions of pseudowires (PWs) include encapsulating service-specific Packet
Data Units (PDUs) arriving at an ingress port and carrying them across a path or tunnel, managing their
timing and order, and any other operations required to emulate the behavior and characteristics of the
service as efficiently as possible.
PW is perceived as an unshared link or circuit of the chosen service. However, there may be deficiencies
that impede some applications from being carried on a PW. These limitations should be fully described
in the appropriate service-specific documents and applicability statements.
Cisco supports standards-based PWE3 as defined by:
• Structure-agnostic TDM over Packet (SAToP), page 1-16
• Structure-aware TDM CESoPSN, page 1-16
• Transportation of ATM Service via MPLS/IP (PWE3/ATM), page 1-17
• Transportation of ATM Service via L2TPv3, page 1-20
A PW is a connection between two provider edge (PE) devices, which connects two attachment circuits
(ACs). An AC can be an ATM virtual path identifier/virtual channel identifier (VPI/VCI) or an T1/E1
link.